Changing router to application port exchange protocol.

The application process needs to request the port from the router instead of the
latter pushing the port before sending a request to the application.  This is
required to simplify the communication between the router and the application
and to prepare the router to use the application shared port and then the queue.

src/nxt_unit.h

@@ -19,6 +19,7 @@
 enum {
     NXT_UNIT_OK          = 0,
     NXT_UNIT_ERROR       = 1,
+    NXT_UNIT_AGAIN       = 2,
 };
 
 enum {